Solids and the liquid phases of drilling muds invade and interact with reservoir formations. This invasion and interaction changes formation permeability and can prevent fluid flow from the reservoir into the wellbore. Damage caused by mud spurt and drilling-mud filtrate can influence log response. The full-length paper describes a novel method to determine the formationdamage potential of drilling muds by evaluating the erosional characteristics of mudcakes formed by different mud systems.
Introduction
Formation damage depends on many factors including formation composition, fluids contacting the formation, fluid and spurt loss characteristics of the contacting fluids, and erosional behavior of deposited mudcake. Drilling and drill-in fluids can cause damage to the reservoir section of a borehole. Mud design criteria that call for maximizing penetration rate and maintaining wellbore stability often overlook the formation- damage potential of drilling mud.
Drilling mud is the first foreign fluid that contacts the reservoir section of a borehole. Drilling mud and drill-in fluids are sources of fine particles that invade the near-wellbore reservoir section. Formation invasion
by drill solids may seal pores and pore throats. Mud-related damage to the producing zone is a function of the erosional characteristics of deposited mudcake.
Formation damage may be caused by physical movement of fine particles into the formation or chemical interactions of solids and filtrate with the formation or a combination of both. The fine particles from drill solids can cause severe formation damage if the mudcake formed on the borehole wall is totally or partially eroded.
Knowledge of mudcake stability and response to erosion are vital in determining how mudcakes will react to mud flow. Additives used in drilling-mud systems can influence mudcake erosional characteristics.
Proper design of drilling and drill-in fluids is extremely important to control spurt and fluid invasion to minimize formation damage. A nonerodible mudcake on the borehole wall reduces drilling-fluid-induced formation damage. Experimental study of the erosional behavior of mudcakes formed by different mud systems can be used as a diagnostic
tool to design, screen, and select the optimum drilling mud or drill-in fluid to drill the reservoir section of a borehole.
Test Setup
The test setup (Fig. 1) consists of an eroding- fluid agitator, a wooden bridge with a circular hole, an erosion cell, and a laboratory jack. The mud agitator consists of a variable speed motor, a speed controller, a rotating shaft, and four fixed fins at the end of the rotating shaft. Additional tools are a fishing hook to fish the mudcake assembly, a hanger to hang the fished mudcake assembly, absorbing tissue papers, and a measuring scale to measure the remaining mudcake weight after each erosion interval. The mud agitator is fixed to the wooden bridge with two belts to keep the agitator shaft centered in the erosion cell. The agitator simulates the hydrodynamic conditions near the drill-collar section of a borehole. An erosive force is created by the rotating fins of the agitator.
The eroding-fluid container is made of transparent plastic to allow observation of the nature of erosion. There is a 22-mm annular clearance after placing the hollow American Petroleum Inst. (API) filtration cell in the center of the eroding cell. The base and annular gaps keep most of the eroded materials away from the mudcake area and prevent their redeposition on the mudcake surface. The 5.5-mm-diameter rotating shaft has widely placed metallic fins. Eroding-fluid volume was
approximately 800 ml. Eroding-fluid temperature was room temperature. Erosional tests were conducted at 925 rev/min.
Mudcake Preparation
Because the objective of the study was to evaluate the erosional characteristics of the mudcake and the damaging potential of the
muds, the muds were designed with common mud additives and the smallest number of mud chemicals. The additives were selected to generate good rheological, filtration, and cake-building characteristics.
Mudcakes were prepared by running a filtration test for 30 minutes in an API filter press. Mudcakes were prepared with and without barite with different salts and fluidloss additives. Anionic, polyanionic, and nonionic fluid-loss additives were used. Mud pH was adjusted to 9.8 to 10 to minimize pH influence on mudcake properties.
Eroding-Fluid Preparation
The eroding fluid should be compatible with the cake matrix fluid to avoid eroding fluid/mudcake interactions. Fresh water was used for freshwater-based bentonite mudcakes, and brine solutions were used for saltwater-
based mudcakes. The fluid was screened after each erosion interval to
remove the eroded particles from the fluid. When the fluid became cloudy because of high concentrations of suspended colloidal particles, the test fluid was replaced with a new batch of eroding fluid.
Experimental Procedure
After mudcake preparation with the API filtration cell, the excess mud was decanted slowly to prevent damage to the mudcake. The API cell containing the mudcake was turned upside down on a sieve for 1 minute to
remove any gel-like loose material from the mudcake surface. The mudcake assembly consisting of the wire screen, rubber gasket, filter paper, and mudcake was carefully removed from the cell. The blank outer edge of filter paper sandwiched between the wire screen and the rubber gasket was fixed firmly with four clips to center the mudcake assembly
in the erosion cell. Eroding fluid was poured into the erosion cell. The erosion cell was placed on the laboratory jack and raised to a level to maintain an 18-mm gap between the filter paper and the end of the rotating shaft.
At the beginning of the test, the eroding fluid was agitated for 2 minutes to wash away the remaining gel-like mud material from the top of the mudcake. The API hollow cylinder was removed; the mudcake was fished with a fishing hook, washed gently to get rid of all loose material, and then hung on a hanger for 2 minutes to remove eroding fluid from the mudcake surface. After that, the assembly was placed on adsorbing tissue
paper for 5 minutes before being weighed. These steps are repeated with a 5-minute eroding-time interval instead of the 2- minute interval. After each 5-minute agitation, the weight of the mudcake was measured.
Shaft speed was kept constant to maintain constant shear stress on the mudcake surface. Mudcake mass was plotted against cumulative erosion time.
Results
The initial part of the weight vs. erodingtime curves, representing less than 5 minutes erosion time is influenced strongly by the presence of the weakly bonded top layer. The last part of the curves, representing 15 to 20 minutes erosion time, is influenced strongly by the adhesive bonds formed
between the mudcake material and the filter paper. The intermediate part of the curves, representing 5 to 15 minutes erosion time, is influenced predominantly by the cohesive bonds formed by the matrix material and was selected to define the erosion coefficient.
Removal of the weakly bonded top layers at the early stage of erosion exposes the wellbonded inner matrix that has similar shear resistance. For this reason, the curves were linear after the initial erosion interval. Erosion coefficients were determined by linear regression.
The freshwater-based bentonite mudcakes and the saltwater-based bentonite mudcakes with different fluid-loss additives showed distinctly different behavior from the saltwater- based bentonite mudcakes. The saltwater- based mudcakes eroded so quickly that no mudcake material was left after 10 minutes erosion time. The erosion mechanics also were different. The saltwater-based mudcakes showed blocky erosion, while the freshwater-based bentonite mudcakes and the mudcakes with fluid-loss additives had smooth surface erosion. Mudcakes containing modified starch showed a combination of surface and blocky erosion.
Freshwater-Based Mudcake.
Fresh-waterbased bentonite mudcake has a lower erosion coefficient than barite/bentonite and NaCl/bentonite/starch mudcakes and has much higher erosion resistance than the saltwater-based mudcakes. Bentonite mud is expected to have lower formation-damage potential due to mud-spurt and mudfiltrate invasion. Bentonite particles have a complex electrical field because of the negative charges at the surface and positive
charges at the edges. In an alkaline mud environment, the result of the interaction forces is repulsive. The net repulsive force during particle deposition results in a mudcake with a high packing density, strong
cohesive bonds, and low permeability. The stiffness of the individual molecular bonds and overlapping of the diffused double layer made the fabric structures resistant to eroding forces. Because of its high erosion resistance, the mudcake formed on the borehole wall will maintain its integrity during drilling.
Incorporating barite in the freshwaterbased bentonite mud made the mudcake, highly erodible. Barite, a chemically inert material, has a much higher density than bentonite, which causes differential settling and creates layering and filling within the mudcake matrix. The cohesive bonds formed by the electrically charged bentonite particles and other molecular forces probably are weaker because of the shielding effect of the barite particles.
Saltwater-Based Mudcakes.
The erosional behavior of saltwater-based mudcakes formed by NaCl/bentonite, potassium chloride/ bentonite, and calcium chloride/bentonite muds indicates that the presence of electrolytes or contamination of bentonite muds by salt or formation brine will increase the formation-damage potential of the mud. Mudcake erosion was so fast that after 10 minutes cumulative erosion time, no appreciable amount of mudcake material was left. Presence of these mudcakes on the borehole wall of a reservoir section will lead to severe formation damage because of
continuous spurt loss and high fluid loss.
Saltwater-Based Mudcakes With Fluid- Loss Additives.
The presence of anionic carboxymethylcellulose (CMC) and polyanionic
cellulose (PAC) in the NaCl/bentonite mud improved the mudcake erosion
resistance significantly. The presence of nonionic starch within the NaCl/bentonite mudcake resulted in a much higher formation- damage potential because of high erosional characteristics of the mudcake. The
presence of fluid-loss additive PAC within the mudcake matrix made the mudcake more erosion resistant compared to the presence of CMC. In the PAC fluid, formation of well-bonded chains or particle garlands makes shearing particles from the mudcake matrix difficult and reduces the plastic-flow tendency of mudcake materials.
Conclusions
- Mudcake erosion by the hydrodynamic action of mud flow can cause formation damage because of continuous spurt loss and high fluid loss.
- The erosional resistance of bentonite mudcake is decreased by the presence of barite and salt within the mud system.
- Mudcake properties, their physical behavior, and their formation-damage potential depend on the physical, chemical, and electrical properties of the mud additives.
